Question

I have the following (simplified) models:

class School(models.Model):
    name = models.CharField(max_length=128)

class UserProfile(models.Model):
    school = models.ForeignKey(School)

class Post(models.Model):
    profile = models.ForeignKey(UserProfile)

What I’d like to do is obtain a list of schools, annotated with the number of posts each school has. For a number of reasons, Post is foreign-keyed to UserProfile instead of School. How would I go about doing this using the Django ORM? I’m guessing I would have to use annotate(), but it does not seem to work when the two models in question are not directly related by a foreign key.

Answer 1

Use lookups that span relationship to Generate aggregates for each item in a QuerySet:

from django.db.models import Count

School.objects.annotate(post_count=Count('userprofile__post'))

Will use such sql:

SELECT "testapp_school"."id", "testapp_school"."name", COUNT("testapp_post"."id") AS "post_count" FROM "testapp_school" LEFT OUTER JOIN "testapp_userprofile" ON ("testapp_school"."id" = "testapp_userprofile"."school_id") LEFT OUTER JOIN "testapp_post" ON ("testapp_userprofile"."id" = "testapp_post"."profile_id") GROUP BY "testapp_school"."id", "testapp_school"."name", "testapp_school"."id", "testapp_school"."name" LIMIT 21;